The present invention relates to communication systems. More particularly, and not by way of limitation, the present invention is directed to a method and arrangement for splicing Highly Rare-Earth-Doped (HRED) optical fibers and dissimilar optical fibers having a large Mode Field Diameter (MFD) mismatch.
In recent years, greater numbers of HRED fibers have been developed for special applications in optical communication systems. Examples include Erbium-Doped Fiber Amplifiers (EDFA), Amplified Spontaneous Emission (ASE) light sources, fiber lasers, and the like. HRED fibers are very attractive due mainly to the excellent performance achieved by a very short length of fiber. For example, by using highly Erbium-Doped Fibers (EDFs) characterized by the peak absorption of 40 dB/m at operating wavelengths, it is possible to reduce the required length of fiber by a factor of 5–10 to get the same performance achieved by traditional optical fibers. EDFs have high power density output, broad and flat gain profile, and low nonlinear effects. Thus, HRED fibers enable the development of more compact and cost-effective communication systems.
The major deterrent to the use of HRED fibers is high splice loss when spliced with dissimilar fibers. A primary reason causing high splice losses for HRED fibers is the initial MFD-mismatch. Because of general demands in the design of systems, HRED fibers are often fusion spliced to dissimilar fibers that have significant difference in MFD, but do not have the rare-earth dopants. A typical example of such a fiber is the Corning SMF28™, and its MFD is approximately twice that of HRED fibers.
In the past, a number of methods have been proposed and developed to reduce the MFD-mismatch during fusion processes. These methods involve mainly an additional thermal treatment applied on the butted portion of fiber having relative smaller MFD (for example, EDFS). The original concept of thermal treatment, developed for the fabrication of optical fiber components, is known as “Thermally-diffused Expanded Core (TEC) fibers” (cf. K. Shigihara, et al; J. Appl. Phys., vol.60, p4293, 1986; and K. Shiraishi, et al; J. Lightwave Technol., vol.8, p1151, 1990).
A TEC method of splicing dissimilar types of fibers is disclosed in U.S. patent Publication No. US 2002/0197027. After fusion splicing of two fibers, the splice point is immediately repositioned so that the additional electric discharge can be applied to the abutted portion of fiber having the relative small MFD. Similar methods using TEC techniques are also disclosed in U.S. patent Publication No. US 2002/0157424 and US 2002/0176673. The methods utilize processes in which the end-faces of two optical fibers are spliced, instead of moving spliced fibers; the additional heating treatment is performed by moving the heating unit (i.e., the electrodes) onto the butted portion of spliced fiber or by heating the splice point asymmetrically with assistance of a heat-board mounted at the vicinity of butted portion of fibers.
The Applicant's studies have found that the previous methods using TEC techniques may work effectively for fibers without rare-earth dopants and/or with low concentration of dopants, but they are not effective for HRED fibers. It was discovered that, for the HRED fibers, besides the problem of initial MFD-mismatch, the major cause of high splice losses can be attributed to fast diffusion of core dopants occurring in the fusion processes. If the initial MFD-mismatch of two fibers is relatively small, the MFD-match of the two fibers may be reached at the very beginning of the main fusion process that is used to make the ordinary splice. Thus, instead of achieving the MFD-match, the additional heat treatment used in the previous methods may lead to an inverted MFD-mismatch (i.e., the HRED fiber that ordinarily has a smaller MFD gets an effective MFD that is much larger than that of the connected fiber). Thus, in the case of light injection from the HRED fiber, the cladding modes are easily excited, and splice losses as high as 0.2–0.5 dB often occur.
Another problem observed in the TEC processes is inconsistent results of splice losses occurring in a sequence of splices. This problem is mainly caused by a phenomenon known as “arc-walk”. Arc-walk refers to the variation of arc-center position from one arc-discharge to the next, due mainly to dynamic changes of deposited particles/layers on the electrodes. The “arc-walk” significantly changes the relative position of heat-distribution at the butted portions of fibers, which in turn varies splice losses.
The problem of arc-walk may be overcome by the method of “arc-recentering” disclosed in International Patent Application No. WO 01/86331. With this method, warm images are taken during fusion processes. The thermal radiation emitted from the heated portion of fiber as well as air discharge is evaluated to determine the position of arc-center. The butted portions of fibers can therefore be repositioned with respect to the arc-center. The method relies on historical data collected in sequences of splices, and thus the accuracy of the method unfortunately decreases with decreasing numbers of splices. Therefore, the method may not be suitable for applications where combinations of fiber types are frequently changed.
A modified method for determining the arc center is disclosed in U.S. patent Publication No. US 2003/0002827. With this method, a preliminary arc-discharge is generated between electrodes without positioning optical fibers at the arc-discharge area. Though the method may be used for estimating the arc position, the error in the estimation of arc position could be quite large, e.g. up to 20 μm. The error is mainly caused by the preliminary arc-discharge itself, since it works as the process of electric cleaning, which in turn changes the condition of particle deposition on the electrodes and causes an additional arc-walk that affects forthcoming fusion processes. Furthermore, in comparison of heat-distribution with and without fiber insertion in the effective area of arc-discharge, it is found that the confinement of heat-distribution could also be varied by the surface/solid plasma excitation caused by energy deposition onto the fibers. Thus, the accuracy of this method could be further degraded due to changes of heat-distribution.
Therefore, there is a need in the art to develop a method and arrangement that can avoid the drawbacks of the existing techniques so that low splice losses for the HRED fibers can be obtained.